When drilling oil and gas wells, it is common to circulate drilling mud downwardly through a string of drill pipe, and up the annular space between the drill pipe and the well bore or casing. The drilling mud is then cleaned and recirculated into the drill pipe. Drilling mud serves a number of important purposes including removal of cuttings, lubrication of the drill bit, and providing sufficient pressure on subsurface formations to prevent blowouts.
It is commonly necessary to either switch from one drilling fluid to another or to remove drilling fluid from the hole entirely and introduce, for example, completion fluid or packer fluid. This may be made necessary because it is desired to switch from water to oil based drilling mud, because a mud change out is needed due to excessively high solids build-up in the well, because drilling mud having a substantially different weight (or other properties) is needed, or because drilling operations are complete. It is, of course, generally desirable to completely remove the old fluid or mud system before introducing the new mud, or other fluid, in any of the above situations. This change of fluids frequently occurs after a certain measured depth has been penetrated, and a string of casing has been run. After the cement has set, and prior to drilling out, is an appropriate time to switch drilling fluids (i.e., oil-based mud to water-based mud), when pipe is run into the hole without a drill bit attached to the bottom hole assembly.
Previously, this has been accomplished by any one of a number of methods, all of which are undesirable because they are excessively time-consuming (and, therefore, costly due to lost rig time), or ineffective because drilling mud remains stuck to the sides of the drill pipe, tubing, casing, or both. If, for example, it is desirable to switch from oil-based mud to water-based mud, a large volume of water-based mud may need to be circulated through the hole and disposed of in order to remove all oil-based mud from the system.
For example, U.S. Pat. No. 4,453,598 describes a method of eliminating drilling mud from a well system which requires circulation of clean water followed by circulation of water treated with a surfactant and alcohol. This method would clearly involve significant amounts of rig time to effectively clean the well. Furthermore, even after the work was complete, there would be no assurance that the casing, drill pipe or tubing would be completely clean.
In U.S. Pat. No. 4,515,212 a casing wiper tool is discussed which is affixed to a conventional drill string and run into the hole. No means of scraping the drill pipe or tubing itself is provided without tripping the pipe out of the hole.
Similarly, U.S. Pat. No. 3,500,933 describes a device which provides molded rubber cups for wiping the casing wall. Again, no means of cleaning the drill pipe or tubing is provided.
In U.S. Pat. No. 1,838,539, the flow of drilling mud past a geologic formation is restricted by withdrawing a piston to lower the pressure to enhance the flow of oil and/or gas into the well bore. While flow is restricted, there is no indication of how a second fluid can clearly replace a prior fluid. Nor is there any indication that patentee's device will scrape the first fluid from the drill string or tubing, and casing.
In U.S. Pat. No. 2,959,224, a casing scraper cleans the inside of casing as well as the outside of the drill pipe. However, the device is comprised of metal components, including, the entire body section, as well as springs, stop rings, nuts, and bolts. The device must therefore be retrieved (which would preclude a fluid transfer) or be milled out. Milling out such a device is very expensive, as additional drill bits are required and a great deal of costly rig time is added.
It is desirable, therefore, to provide a well casing wiper that can be utilized to effectively scrape both the outside of a drill string or tubing and the inside of the casing, and especially to separate or prevent the mixing of, for example, different drilling fluids.